1. Field of the Invention
This invention relates to leak testing that uses a test gas, and more particularly to leak testing of an automotive vehicle fuel tank before it is installed in a vehicle using helium as the test gas.
2. Background Information
One type of equipment that is in use for leak testing mass-produced automotive fuel vehicle tanks before they are installed in a vehicle comprises a test chamber having a hood that opens to allow a fuel tank to be placed within a test chamber space, that then closes during the test, and that finally re-opens after the test to allow the tested fuel tank to be removed. After the hood closes, helium is introduced into the interior of the fuel tank in a manner that achieves sufficient helium concentration and pressure within the tank for the test to proceed. The helium is introduced into the tank through an opening in the tank wall, such as a fill spud, via a conduit that terminates in an end fitting that associates with the tank opening in a sealed manner.
For achieving helium concentration and tank pressure suitable for a test to proceed, the helium is introduced via several pressure/vacuum cycles (three such cycles for instance) that replace a significant portion of the tank air with helium. A pressure of about two psi (pounds per square inch) and a helium concentration of about 40 percent, which are satisfactory values for a test, can be achieved in this manner. While the helium is being introduced, fresh air is circulated through the test chamber space external to the fuel tank for purging any residual helium that may have accumulated under the hood as a result of testing of a previous tank.
Once the purging has concluded and the proper conditions for commencing a test are present within the tank interior, a mass spectrometer that is communicated to the test chamber space begins sampling the background air in that space. Leakage from the tank is evidenced by the mass spectrometer sensing an increase in the helium concentration. The amount by which the mass spectrometer reading increases during a test correlates with a hydrocarbon emission leak rate in terms of mg/day (milligrams HC per day).
The test equipment just described is limited in ability to detect leaks smaller than a certain size. For example, it is believed that the equipment may be incapable of accurately sensing leaks smaller than an equivalent leak that correlates with a hydrocarbon emission leak rate of 100 mg/day. Increasingly stringent regulations may render that test equipment obsolete for determining fuel tank compliance, not necessarily because of a total inability of the equipment to detect the even smaller leaks, but because the ability to do so will require longer test times and more helium. Those factors, or the alternative of adding more test machines of the same kind, add cost to the testing of mass-produced vehicle fuel tanks.
Accordingly, there is a perceived need for test equipment that is capable of measuring even smaller leaks in mass-produced fuel tanks with improved accuracy and cost-effectiveness.
It is believed that limitations in the ability of the existing test to efficiently measure even smaller leaks are due to one or more factors, such as an inherent limit in the sensitivity of the mass spectrometer used in the equipment, the amount of time and helium required to charge a tank that is to be tested, and leakage between the chamber space and its ambient surroundings when the hood is closed.
A more sensitive mass spectrometer is apt to add more cost to the equipment, and therefore measures that can improve the equipment without such added expense would be desirable.
It would also be desirable if the amount of helium uses for a test could be reduced, and if the test duration could be minimized.
Because the test enclosure comprises a hood that seals against a base, any imperfection in sealing between them may become a source of error in test measurement, possibly even giving a false indication of a leak if residual helium in the ambient surroundings were to intrude into the chamber due to faulty sealing of the closed hood to the base. Because a certain minimum helium concentration must be present in the chamber space external to the tank before the mass spectrometer can even give a reading, leakage in the opposite sense, i.e. from the chamber space to the ambient surroundings, may to some extent mask a leak.
Because an entire fuel tank is placed within an enclosed test chamber, specific location of a leak is not likely to be determined using the equipment and method described above. If a defective manufacturing operation that creates a reoccurring leak in fuel tanks is present in a mass-production process, such as at a welding station that welds a component to a tank, the ability to promptly identify the location of that leak may allow early correction of the faulty operation, and obvious savings, when contrasted with tardier correction.
Accordingly, it is also considered desirable for test equipment to possess a capability for disclosing the location of a leak in a tank. It would also be beneficial if fuel tanks could be tested with equipment that does not require placement of an entire tank within a test enclosure as described above.
A preliminary novelty search in connection with this invention developed the following U.S. Pat. Nos. 3,616,680; 3,813,923; 3,842,659; 1,949,596; 4,576,038; 4,577,490; 4,601,194; 4,791,805; 4,862,731; 4,998,435; 5,309,752; 5,375,457; and 5,509,296; and Russian Patent Document No. 232,567.
Certain of those patents are specific to testing automotive fuel vehicle fuel tanks, for example U.S. Pat. Nos. 4,791,805; 4,862,731; and 5,509,296. Certain patents disclose placement of the entire article that is being tested for leakage within an enclosure. Other patents disclose placement of an inverted cup against the exterior of an object in covering relation to an underlying area of the object being tested for leakage. Helium is a commonly used test gas, and mass spectrometers are commonly used for measuring helium concentration.